Adult stem cells are preferred source for cells to be used in cell therapy methods for regenerating and repairing tissues, as they are not plagued with the problems of induced pluripotent stem cells or embryonic stem cells. Furthermore, when autologous stem cells can be used, transplant rejection and graft-versus-host disease can be avoided without the use of toxic immunosuppressive therapy. Adipose stem cells are particularly desirable given the ease of harvesting with minimally invasive procedures and the high percentage of stem cells. Human adipose tissue is a source of adult stem cells that have been shown to have differentiation capacities, being capable of generating myogenic, osteogenic and endothelial lineages (see, e.g., Baer and Geiger, Stem Cells Int. 2012; 2012:812693). Homogeneous populations of cells are desirable for use in clinical methods, but previous methods (which typically involve a culturing step) produce heterogeneous mixtures of cells with no specific prospective molecular identity.
Adipose tissue is comprised of the parenchymal cells (adipocytes) and their stomal vascular support (extracellular matrix, vasculature). Adipocytes can be classified into “white” “brown” or “brite/beige” depending on their functional role. White adipocytes primarily store excess energy in the form of triglycerides, stored in a single large droplet within their cytoplasm. Brown adipocytes primarily burn fat for the purpose of providing heat to protect organs from cold exposure. “Brite” (Brown-in-white) or “beige” adipocytes, are similar to white adipocytes in that they can store fat in droplets within the cytoplasm, but resemble brown adipocytes in that they can burn fat. Brown fat is present in hibernating animals and human infants, but is absent in human adults. However, brite/beige cells remain and can be found in the supraclavicular region, and interspersed within white adipocytes. Mice also have beige/brite adipocytes interspersed within their white adipose tissue depots. Beige/brite adipocytes are highly metabolic, and their presence is correlated with lean, insulin sensitive phenotypes. This has suggested that strategies to increase brite/beige cells would be useful for the treatment of obesity-associated diseases.
There is much interest in identifying both white and beige/brite cell progenitors in order to expand them and using them for therapeutic purposes. Uses for white adipocytes include grafting for plastic and reconstructive surgery. Uses for brite/beige adipocytes include grafting to increase metabolic rate and prevent obesity and complications. Isolation of brite/beige progenitors will also allow the development of assays to screen for drugs that will enhance their proliferation and differentiation. Approaches to identify white adipocyte progenitors have included isolation by collagenase digestion of all cells comprising the stromal vascular fraction, and selection of cells through plating on plastic. This produces a heterogeneous population of cells, which differentiate variably and poorly. Further enrichment of progenitors using fluorescence-activated cell sorting has also been attempted, but results in very low cell yields and remaining heterogeneity. Brite/beige progenitors have never been identified by this procedure. Thus, a better method to expand and identify human adipocyte progenitors is necessary.